The invention relates to a transistor, a memory cell array and a method for forming and operating a memory device with at least one memory cell, in particular a resistively switching, e.g., a phase change memory cell, and to a memory device.
Conventional memory devices, in particular semiconductor memory devices, can be differentiated into a first group of functional memory devices, e.g., PLAs, PALs, etc., a second group of table memory devices, e.g., ROM devices such as PROMs, EPROMs, EEPROMs, flash memories, etc. Furthermore there is a third group of RAM devices, such as DRAMs and SRAMs.
Recently “resistive” or “resistively switching” memory devices have also become known, e.g., so called Phase Change Memories (“PCMs”) or Conducting Bridge (CB) memories or magnetoresistive memories (MRAM) or resistive RAM (RRAM).
In a “resistive” or “resistively switching” memory cell, an “active” or “switching active” material, which usually is positioned between two suitable electrodes, i.e. an anode and a cathode, can be switched between a conductive and a less conductive state by an appropriate switching process. The conductive state can be assigned a logic one and the less conductive state can be assigned a logic zero, or vice versa, which may, for instance, correspond to the logic arrangement of a bit.
For phase change memories (PCRAMs), for instance, an appropriate chalcogenide compound, for example Ge—Sb—Te (GST) or an In—Sb—Te compound, may be used as a “switching active” material that is positioned between two corresponding electrodes. This “switching active”, e.g., the chalcogenide material, can be switched between an amorphous and a crystalline state, wherein the amorphous state is the relatively weakly conducting state, which accordingly can be assigned a logic zero, and the crystalline state, i.e. a relatively strongly conductive state, accordingly can be assigned a logic one. In the following this material will be referred to as the switching active material.
To achieve a change from the amorphous, i.e. a relatively weakly conductive state of the switching active material, to a crystalline, i.e. a relatively strongly conductive state, the material has to be heated. For this purpose a heating current pulse is sent through material, which heats the switching active material beyond its crystallization temperature thus lowering its resistance. In this way the value of a memory cell can be set to a first logic state.
Vice versa, the switching material can be heated by applying a relatively high current to the cell which causes the switching active material to melt and by “quench cooling” the material can brought into an amorphous, i.e. relatively weakly conductive state, which may be assigned a second logic state, that is to reset the first logic state.
Various concepts have been proposed for PCRAM cells, for example from S. J. Ahn, “Highly Manufacturable High Density Phase Change Memory of 64 MB and Beyond”, IEDM 2004, and H. Horii et al. “A novel cell technology using N-doped GeSbTe films for phase change RAM”, VLSI, 2003, and Y. N. Hwang et al “Full integration and reliability evaluation of phase-change RAM based on 0.24 um-CMOS technologies”, VLSI, 2003, and S. Lai et al “OUM—a 180 nm non-volatile memory cell element technology for stand alone and embedded applications”, IEDM 2001, or the edge contact cell by Y. H. Ha et al “An edge contact cell type cell for phase change RAM featuring very low power consumption”, VLSI, 2003.
To be cost competitive a small cell size is required allowing a high density of memory cells in a memory cell array.
A disadvantage of the proposed memory cells is the use of planar array transistors or transistors having the source/drain contacts in the same horizontal plane, for example FinFETs. Such a design prohibits shrinking the cell size below 6F2 for geometrical reasons, because the size of a cell includes the area needed for the transistor to select the cell.
US2005/0001257A1 discloses a DRAM memory cell having vertical transistor cells formed in a substrate including lower source/drain regions connected to a common connection plate. Upper source/drain regions of the transistor cells impart a contact connection to a storage capacitor. The array of transistor cells is formed by wordline trenches, wherein the wordlines in the trenches form gate electrodes of the transistors, and by isolation trenches (STI) running perpendicular to the wordline trenches. A disadvantage of the proposed structure is the need to accommodate and form two electrically isolated spacer wordlines in one wordline trench.
Consequently it is desirable to provide a novel design of a selection transistor for a memory device with a plurality of memory cells, in particular phase change memory cells, and a corresponding method for forming the transistors avoiding the abovementioned problems.
For these and other reasons, there is a need for the present invention.